Optical mark and optical character readers, which employ line scan cameras having an array of photosensitive elements, develop analog output signals on a line by line basis of a scanned document which provide an indication of the presence or absencee of characters or marks on the basis of light reflective from or transmission through the document.
The black/white signature of a scan line, or sweep of a line scan camera, across a document, such as a student's test answer sheet, a voting ballot, an order entry form, etc., produces an analog video output signal from the camera. The analog signal is typically applied to a video normalizer circuit to initially provide a black level compensation and determine the gray level of the discrete picture elements (pixel) of the discrete portion of the analog signal corresponding to the individual photosensitive elements of the line scan camera. The gray level analog output signals are supplied to an analog to digital converter which generates a two bit output identifying the gray level of each picture element to determine if the gray level corresponds to an accepted valid mark or character on the document, or is indicative of an erroneous mark. This multi-level mark, or character, density discrimination provided by the two bit output of the analog to digital converter permits electronic differentiation between an inadvertent pencil mark and a legitimate mark or character appearing on the document.
The utilization of a line scan camera and digital signal processing circuitry for deriving intelligence from a document is described in detail in pending U.S. patent application Ser. No. 457, filed Jan. 2, 1979, entitled, "Optical Reading System" by J. V. McMillin and D. W. Schroeder, now U.S. Pat. No. 4,300,123, which is assigned to the assignee of the present invention and incorporated herein by reference.
In order for the typical state of the art system to operate effectively, the system recognizes marks appearing in data entry mark locations only when the marks are preceded by a special printed document mark called a scan mark. Scan marks are used for document verification and registration.
Typically, a single scan line of the line scan camera is employed to verify the presence of a scan mark and thus alert the signal processing circuitry to the data or character information to follow from subsequent scan lines of the document. A single scan line verification of the presence of a scan mark, however, renders the system vulnerable to false verification due to the presence of erroneous marks on the document. Inasmuch as the application of the optical scanning system often involves sensitive and important information such as student tests, census information, voter ballots, it is important to minimize erroneous responses such as the false verification of a document scan mark.
There is disclosed herein with reference to the accompanying drawings a unique multi-line scan mark verification technique.